A List of Bad Bacteria

Bacteria are the oldest and most abundant life form on Earth. These microscopic, single-celled organisms live in all environments -- including in and on the human body. A study published in May 2016 in the "Proceedings of the National Academy of Sciences of the U.S.A." estimated there may be as many as 1 trillion different types of bacteria. Most bacteria are harmless to humans, and many that live in and on the body serve beneficial functions. Certain bacteria, however, have the capacity to cause infectious diseases and conditions. Disease-causing, or pathogenic, bacteria can be categorized based on their shape and staining pattern.

Gram-Positive Cocci

Bacteria are broadly categorized as Gram positive or negative based on their staining characteristics. This distinction is important as it reflects differences in the bacterial wall, which influences its susceptibility to different antibiotics. Certain types of Gram-positive cocci, or spherical bacteria, are the leading pathogens among humans, including: -- Staphylococcus aureus -- Streptococcus pneumoniae -- Streptococcus pyogenes, also known as group A streptococcus

Gram-Negative Cocci

Among the Gram-negative cocci, two specific bacterial species stand out as important human pathogens. Neisseria gonorrhoeae, also known as gonococcus, is the bacteria responsible for the sexually-transmitted infection gonorrhea. Approximately 820,000 new gonorrhea infections occur each year in the U.S., according to the Centers for Disease Control and Prevention. Although gonorrhea is not immediately life-threatening, it can lead to serious health problems such as infertility in women and infectious arthritis. Gonorrhea can also cause serious complications in babies born to mothers with the infection.

Neisseria meningitidis, or meningococcus, is a leading cause of bacterial meningitis -- a life-threatening infection of membranes brain and spinal cord, which can spread throughout the body via the bloodstream. CDC recommends meningococcal vaccination for all preteens and teens, and certain adults at risk for contracting the infection.

Bacillus anthracis is the bacteria responsible for anthrax, a feared bioterrorism threat. A related species called Bacillus cereus causes food poisoning mediated by toxins the bacteria produce. Listeria monocytogenes causes listeriosis, a serious infection caused by eating food contaminated with the bacteria. With listeriosis, the bacteria typically spreads from the digestive system to other parts of the body. Listeriosis, which is potentially life-threatening, primarily affects newborns, seniors, pregnant women and other people with a weakened immune system.